As stated in Using Top-Level Containers article, the manu bar is managed by the Root Pane:
Each top-level container relies on a reclusive intermediate container
called the root pane. The root pane manages the content pane and the
menu bar, along with a couple of other containers. You generally don't
need to know about root panes to use Swing components. However, if you
ever need to intercept mouse clicks or paint over multiple components,
you should get acquainted with root panes.
A menu bar is tipically placed at the top of a window, so if it would be placed in the content pane then the Layout managers would have to deal with this particular component. By segregating the menu bar from the content pane then the responsibility of laying out the menu bar and the rest of components is well divided. In addition, the menu bar location is fixed and this component is not dragabble so this can be laid out in a different way than the rest of components.
About the JToolBar this component is draggable and you may have several of them, placing one above another one and rearranging their order as you wish. So this component is totally different of a menu bar and makes sense it be placed in the content pane just as a text field or another regular component.
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